[DebugInfo] Move Value struct out of DebugLocEntry as DbgValueLoc (NFC)

Since the DebugLocEntry::Value is used as part of DwarfDebug and
DebugLocEntry make it as the separate class.

Reviewers: aprantl, dstenb

Reviewed By: aprantl

Differential Revision: https://reviews.llvm.org/D63213

llvm-svn: 363246